In conventional temperature detection devices, the fact that the characteristic curve of a diode has a strong temperature dependence is utilized.
Integrating such temperature detection devices in measuring devices, which include a sensor for detecting a variable to be measured and in which the output signal of the sensor is a function of the ambient temperature, is also conventional. Examples of such sensors are micromechanical pressure, acceleration, or yaw-rate sensors. For their usage in corresponding applications, it is necessary that a value, which is independent of the ambient temperature, of the variable to be measured is calculated.
In many micromechanical sensors, the measured variable, for example, pressure, acceleration, or yaw rate, is detected by the change in piezoelectric resistances or capacitances and converted into an electrical voltage.
The ambient temperature represents a disturbance variable, which has an influence on the electrical output voltage of the sensor element. To be able to compensate for the influence of the ambient temperature within an integrated circuit, it must also be detected. Resistors or diodes are used to detect the temperature. Diodes are typically powered using a constant current to generate a temperature-dependent voltage.
The signal processing for the purposes of temperature compensation, amplification, or linearization is implemented employing digital integrated circuits. Analog-digital converters are used to convert the analog electrical voltages of diode and sensor element into a digital numeric value which is processable by a computer unit.
In order to minimize the conversion error due to variations in the supply voltage for the pressure signal, the pressure sensor and the analog-digital converter are operated using the same supply voltage. The sensitivity of the pressure sensor thus decreases upon reduction of the supply voltage, and the sensitivity of the analog-digital converter therefore proportionally increases inversely, whereby the digital numeric value at the output of the analog-digital converter remains unchanged (ratiometric measurement).
The diode is operated using a constant current which is largely independent of the supply voltage, so that the output voltage of the diode is also largely independent of the supply voltage. If the supply voltage drops, the diode delivers an unchanged output voltage, while the sensitivity of the analog-digital converter rises, however, and results in a higher digital numeric value at the output of the analog-digital converter. A misinterpretation of the diode output voltage and an incorrect ascertainment of the measured value after the temperature compensation in the signal pathway thus occur.